Screening of a natural compound library for potential protein kinase inhibitors

Aim: Identification of new protein kinase inhibitors from a natural compound library.

Origin: The compounds are provided by the National Cancer Institute and the NIH (USA) under the „Developmental Therapeutic Program“ (DTP).

Screening methods: Luminometric and radioactive detection assays.

Results: We have screened 230 natural compounds from the DTP Natural Product Set for their ability to inhibit protein kinases CK2 and AKT as targets.

Although the chances to find a specific and potent inhibitor from a set of only 230 compounds are fairly limited, yet we have identified several compounds which show an IC₅₀ of <10 micromolar towards CK2. The initial screening was performed using a luminometric test which allows the screening in a 96 well format. Positive compounds were confirmed using the classical radioactive test. The most potent compound identified, specifically inhibiting protein kinase CK2, was resorufin with an IC₅₀ of 1.4 micromolar towards CK2. It did not inhibit any other protein kinases tested in a panel of 25 kinases (in contrast to emodin, an established CK2 inhibitor, whose IC₅₀ is in the same range. However, in contrast to resorufin, emodin is fairly unspecific since it inhibits other protein kinases such as Chk1, Chk2, AMPK, DYRK2, GSK3beta, JNK2, MLK1, PI3K, RIPK as well. Treatment of various cell lines with resorufin showed inhibition of CK2 in intact cells and induction of apoptosis. Resorufin may therefore be considered as a valuable scaffold for the development of more efficient and specific inhibitors of protein kinase CK2.